The most familiar and widely adopted types of filter bags on the market today are those with one or two lobes, or product containing segments, of substantially flattened shape and formed from filter paper wrapped around the charges of infusion product to form a tube.
Other types of filter bags also known and used, although less widespread than the commercially more popular single or two-lobe bags, are those of three-dimensional shape, such as the pouch-like bag described in United States patent U.S. Pat. No. 2,187,417, or those of tetrahedral shape, hereafter also referred to as “pyramidal” for convenience, as described in United States patent U.S. Pat. No. 4,290,521.
In one of its aspects, the invention is specifically concerned with the steps of forming the pyramidal filter bag, filling it with infusion product and finally sealing it.
A prior art machine for making pyramidal filter bags is described in United States patents U.S. Pat. No. 4,290,947 and U.S. Pat. No. 5,548,947. This machine comprises a station for continuously feeding a web of filter paper, and a station for forming a continuous tube extending vertically; a dosing unit for filling doses of infusion product into the vertical tube; sealing/folding stations for making seals/folds and designed to make the transversal seals in succession and again working in a vertical direction, by sealing the tube together with the folds necessary to make the pyramidal shape.
In practice, the pyramidal filter bag according to the above mentioned United States patents is made in a vertical production line using a process that includes the steps of positioning and longitudinally sealing the filter bag to form the tube; making a first transversal closure at the lower end of the tube; filling a dose of product into the lower end of the tube, depositing it by gravity; making a second transversal closure, at 90° to the previous one, so as to form the pyramidal shape; and, finally, cutting the filter paper tube to form the pyramidal filter paper bags in succession.
This process, however, has notable disadvantages due especially to the dosing unit which meters and fills the infusion product into the tube at the dosing station of the pyramidal filter-bag making machine.
Indeed, the dosing unit normally comprises a hollow cylindrical element into which the doses of infusion product are metered and which subsequently releases the doses one after the other into the lower end of the filter paper tube. This cylindrical element extends vertically and has a considerable length.
Dosing operations within the hollow cylindrical element have inherent speed limitations on account of their complexity and therefore significantly slow down the operation of the tea-bag making machine as a whole. Further speed limitations are due to the complex sealing operations by which the end of the filter paper is closed to give the filter bag its pyramidal shape.
Thus, machines that make pyramidal filter bags of the type described above operate only at limited speeds and, hence, cannot meet current market demands for high speeds and large volumes of production.
The present invention therefore has for an object to provide a machine and a related filter bag production method that overcome the above mentioned drawbacks of prior art.